Diagnostic note for invalid @_lifetime annotations on inout params

Users commonly try to write a lifetime dependency on an 'inout' parameters as:

    @_lifetime(a: &a)
    func f_inout_useless(a: inout MutableRawSpan) {}

This is useless. Guide them toward what they really wanted:

    @_lifetime(a: copy a)

Fixes rdar://151618856 (@Lifetime(..) gives inconsistent error messages)

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -910,17 +910,21 @@ class LifetimeDependenceChecker {
       if (!paramDeclAndIndex.has_value()) {
         return std::nullopt;
       }
-      auto lifetimeKind =
-        getDependenceKindFromDescriptor(source, paramDeclAndIndex->first);
+      auto *param = paramDeclAndIndex->first;
+      unsigned sourceIndex = paramDeclAndIndex->second;
+      auto lifetimeKind = getDependenceKindFromDescriptor(source, param);
       if (!lifetimeKind.has_value()) {
         return std::nullopt;
       }
-      unsigned sourceIndex = paramDeclAndIndex->second;
       if (lifetimeKind == LifetimeDependenceKind::Scope
-          && paramDeclAndIndex->first->isInOut()
+          && param->isInOut()
           && sourceIndex == targetIndex) {
         diagnose(source.getLoc(),
                  diag::lifetime_dependence_cannot_use_parsed_borrow_inout);
+        ctx.Diags.diagnose(source.getLoc(),
+                           diag::lifetime_dependence_cannot_infer_inout_suggest,
+                           param->getName().str());
+
         return std::nullopt;
       }
       bool hasError =

test/Sema/lifetime_depend_infer.swift

@@ -566,6 +566,7 @@ struct NonEscapableMutableSelf: ~Escapable {
   mutating func mutatingMethodNoParamCopy() {}
 
   @_lifetime(self: &self) // expected-error{{invalid use of inout dependence on the same inout parameter}}
+                          // expected-note @-1{{use '@_lifetime(self: copy self) to forward the inout dependency}}
   mutating func mutatingMethodNoParamBorrow() {}
 
   mutating func mutatingMethodOneParam(_: NE) {} // expected-error{{a mutating method with a ~Escapable 'self' requires '@_lifetime(self: ...)'}}
@@ -586,10 +587,17 @@ struct NonEscapableMutableSelf: ~Escapable {
 
 // Unable to infer an 'inout' dependency. Provide valid guidance.
 //
-func f_inout_no_infer(a: inout MutNE, b: NE) {} // expected-error{{a function with a ~Escapable 'inout' parameter requires '@_lifetime(a: ...)'}}
-// expected-note @-1{{use '@_lifetime(a: copy a) to forward the inout dependency}}
+func f_inout_no_infer(a: inout MutNE, b: NE) {}
+// expected-error @-1{{a function with a ~Escapable 'inout' parameter requires '@_lifetime(a: ...)'}}
+// expected-note  @-2{{use '@_lifetime(a: copy a) to forward the inout dependency}}
 
 // Invalid keyword for the dependence kind.
 //
 @_lifetime(a: inout a) // expected-error{{expected 'copy', 'borrow', or '&' followed by an identifier, index or 'self' in lifetime dependence specifier}}
 func f_inout_bad_keyword(a: inout MutableRawSpan) {}
+
+// Don't allow a useless borrow dependency on an inout param--it is misleading.
+//
+@_lifetime(a: &a) // expected-error{{invalid use of inout dependence on the same inout parameter}}
+                  // expected-note @-1{{use '@_lifetime(a: copy a) to forward the inout dependency}}
+func f_inout_useless(a: inout MutableRawSpan) {}